Contents

The novel is narrated by the protagonist Jack Forman, who is an unemployedsoftwareprogrammer who used to work for a company called Media Tronics but was fired for discovering an internal scandal. As a result, he is forced to take the role of a house husband while his wife Julia serves as a high ranking employee at a nanorobotics company called Xymos. Julia claims that she is working on a new piece of revolutionary imaging technology with her company, which takes up most of her time and makes her grow distant to Jack and her family. He starts believing that during her long hours away from home she is having an affair, and becomes watchful of her changes.

One night Julia comes home late and shows Jack a video of her demoing the Xymos nanobots. In the video, the nanobots are put into a human test subject, and video from inside the body is broadcast in real time; Jack is impressed, but becomes more suspicious of her straying when he notices the video was not made on the same day as she said it was. Later in the night, their baby girl Amanda awakens in agony as her body turns red from an unknown cause. Jack takes her to the emergency room but the doctors cannot identify the cause of her pain. She is given an MRI for more examination, then suddenly her pain stops and the skin changes disappear. Bewildered and exhausted he returns home to a strangely indifferent Julia who leaves in a hurry, claiming to go on urgent business trip.

Strange events happen that lead Jack to suspect something bigger is going on with Julia as their son Eric claims he saw silver men cleaning the house in the middle of the night, a strange device found underneath Amanda's bed that wasn't there before, memory chips on their home's electronics turning to dust, Julia not only turning distant but also abusive to her children, and Julia driving away one night with an unidentifiable passenger in her car. On that same night, Julia is injured in a car accident, and Jack is offered a job by Xymos, because the project manager and a good friend of Jack, Ricky, is having software issues with the nanobots.

Jack is taken to the Xymos research facility in the Basin Desert. Jack is given a tour of the lab and meets the programming team. He is shown a very complicated machine used to make the nanobots from bacteria. Ricky refuses to show Jack the source code for the nanobots, and later Ricky claims that building contractors failed to properly install filters in a certain vent in the building. As a result, hazardous elements such as the assemblers, the bacteria, and the nanobots were blown into the desert, evolving and eventually forming autonomous swarms. These swarms appear to be clouds of solar-powered and self-sufficient, reproducing and evolving rapidly. The swarms exhibit predatory behavior, attacking and killing animals in the wild, using code that Jack himself worked on. Most alarmingly, the swarms seem to possess rudimentary intelligence, the ability to quickly learn and to innovate. Jack also learns that Julia helped teach the swarms to improve their intelligence and become more benign, but regressed when she left. The swarms tend to wander around the fab plant during the day but quickly leave when strong winds blow or night falls.

The nanoswarm kills a rabbit outside the complex, and Jack goes outside with Mae to inspect. They find that the rabbit died of suffocation resulting from the nanobots blocking its bronchial tubes. While Mae goes inside for equipment, Jack is attacked by the swarms. He barely manages to get through the airlock inside the lab before falling unconscious from anaphylactic shock.

Persuaded by Jack despite Ricky's protests, the team decides to destroy the swarm or else risk it reproducing exponentially and become a grey goo plague that could endanger humanity. They believe that the swarm must have nested in the desert to reproduce. They attempt to find this nest by tagging the swarm with radioactive isotopes and following them back to their nest at night. Under the cover of a strong wind that forces the swarms to remain dormant, the team goes outside to a storage shack to find the isotopes and build a spray device. However, as the wind dies down, four swarms attack the shack and eventually kill David and Rosie. The rest of the team are forced to take shelter in the cars parked outside.

The swarms find a way to enter the cars, but not long before the wind picks up in speed again. Jack and Mae manage to escape to the lab before losing consciousness, but Charley falls unconscious outside his car after he sprays his swarm with the isotope. Bobby, Vince and Ricky refuse to go outside and help Charley. Jack, dizzy and nauseous, goes back out again to save Charley as the swarms attacks again. Using a motorbike found in David's car, Jack manages to get himself and the semi-conscious Charley to the safety of the airlock before he falls unconscious again.

As night falls, Jack, Mae and Bobby set out to find the swarms. While searching for them, they discover that the swarms are moving the now deceased Rosie through the desert. The team is also shocked to discover that the swarms can replicate the physical features, perceptions, and motions of humans when they see the swarms form replicas of Ricky, David, and Rosie. They follow the body to find the swarms nesting in a cave. As some of the Ricky-swarms come out of the cave after them, a Xymos helicopter arrives and traps the swarms inside the cave using its powerful draft. Mae and Jack then venture into the cave and proceed to exterminate the swarm, their nest and their organic assembly plant (which looks very similar to the original Xymos assembly plant) using explosive thermite caps. They return to the Xymos plant, exhausted.

At the plant, Jack, Mae and Bobby are enthusiastically greeted by Julia, who discharged herself from the hospital and was brought in by the chopper. Julia's behavior seems to be extremely aberrant: She seems to pay heed to nothing other than trying to entice Jack and kissing him, even when Charley is found dead in the locked communications room with a swarm flying around him and the communication links cut. Jack cannot understand how the swarm got inside the rigorously protected airtight building, why Charley would have disabled the facility's communications, or why Julia and Ricky seem to be coming up with various out-of-character ways of how he died.

Mae discovers security footage of when they were in the desert. To Jack's horror, the video not only reveals that Julia and Ricky had an affair but also shows how Charley engaged in a vicious fight with Ricky and Vince. All of them end up in the communications room where Julia kisses a subdued Charley, injecting a stream of swarm into his mouth while Ricky sabotages the communication systems and they leave Charley to die from the swarm.

Eventually, Jack and Mae realize that everyone in the facility except themselves have been infected by a symbiotic version of the nanobot swarms. These nanobots, although evolved alongside the other swarms, do not show aggressive predatory behavior. Instead, while they seem to invigorate their hosts' physical statistics and their emotional perceptions, they slowly devour and take over their hosts to produce more nanobots, initially affecting their decisions and then controlling them, while allowing them to travel and contaminate others.

Jack comes up with a plan to destroy this new strain. Mae and Jack drink vials containing a form of phage that kills the nanobot-producing E. coli bacteria. The phage would protect them from infection. Jack then proceeds to take a sample of the phage and pour it into the sprinkler system and drench everyone with it. He tricks Mae into alerting Julia and the infected team. They set out to stop Jack. In the vicious struggle that ensues, Jack is captured and thrown into a magnetic chamber, with Ricky threatening him to reveal his plan or kill him by destroying the magnets. Jack feigns surrender when Julia walks in to interrogate him, but Jack then reactivates the magnetic chamber with Julia inside, remembering the incident with Amanda in the MRI. Julia's body disintegrates as a swarm is pulled away by the magnetic field to temporarily reveal the real Julia, who has slowly been consumed by the parasitic swarm. Before the swarm can repossess her body, Julia begs Jack to forgive her for putting him in danger, that she loves him, and tells him to stop the swarms and save their children from the same fate as they have been infected too. Motivated, Jack runs to the roof, evades the infected-team and finally manages to place the sample into the sprinkler system.

In order to prevent the sprinkler system from triggering, infected-Ricky disables the plant's safety network. However, this is exactly what Jack wants, as Mae has already allowed the phage into the assembly line, causing the phage to reproduce rapidly. The assembly line is rapidly overheating because of the no longer active safety system. If infected-Ricky and infected-Julia don't turn on the safety system the assembly line will burst, filling the lab with the phage. The infected-team, who are now doomed either way, choose to re-activate the safety network and get drenched with the phage. Jack and Mae escape the facility in a helicopter shortly before the facility explodes due to a methane gas leak combined with thermite Mae has placed in the building. After returning home, Jack infects all his children with the phage to eradicate the potential nanobot infestation. Mae calls the U.S. Army and sends a sample of the phage to her lab.

Jack puts together all the missing links. The corrosion of the memory chips his home's electronics as well as Amanda's skin rash were caused by nanobot assemblers that infected-Julia spread. The MRI's strong magnetic field detached the assemblers from her. Knowing this, Julia called in the Xymos special team to scan Amanda's room. The person who Jack spotted in Julia's car was in fact the Ricky-swarm. Xymos intentionally released the swarm into the desert so that it would evolve to stay in a cohesive group in the wind.

The "Swarm" – Any of the many predatory clouds of nano-machines that "eat" animals serving as an antagonistic force in the novel. "Eating" means suffocating the animals and growing bacteria on them, thereby producing more nano-machine assemblers, and more nano-machines. A notable aspect of the swarm is its capacity for fully Lamarckian evolution, as each cloud's members can effectively choose exactly which aspects are to be transmitted or modified down into the next generation through manipulation of the E. coli bacteria used to produce the new robots.

Jim Holt, writing for the New York Times, found the book "absurd" but exciting, and said that he "kept turning the pages feverishly".[2]

Peter Guttridge, writing for The Observer, said that it finds Crichton "doing what he does best". In that he takes "the very latest scientific advances" and shows "their potentially terrifying underbelly".[3]